Acute cerebral ischemia in rats studied by Carr-Purcell spin-echo magnetic resonance imaging: Assessment of blood oxygenation level-dependent and tissue effects on the transverse relaxation

Acute cerebral ischemia has been shown to be associated with an enhanced transverse relaxation rate in rat brain parenchyma, chiefly due to the blood oxygenation level-dependent (BOLD) effect. In this study, Carr-Purcell R ₂ (CP R₂), acquired both with short and long time intervals between centers of adiabatic π-pulses (τ_CP), was used to assess the contributions of BOLD and tissue effects to the transverse relaxation in two brain ischemia models of rat at 4.7 T. R_1p and diffusion MR images were also acquired in the same animals. During the first minutes of global ischemia, the long τ_CP R₂ in brain parenchyma increased, whereas the short τ_CP R₂ was unchanged. Based on the simulations, and using constraints of intravascular BOLD effect on parenchymal R₂, the former observation was ascribed to be due to susceptibility changes arising in the extravascular compartment. R _1p declined almost immediately after the onset of focal cerebral ischemia, and further declined during the evolution of ischemic damage. Interestingly, short τ_CP CP R₂ started to decline after some 20 min of focal ischemia and declined over a time course similar to that of R_1p, indicating that it may be an MRI marker for irreversible tissue changes in cerebral ischemia. The present results show that CP R ₂ MRI can reveal both tissue- and blood-derived contrast changes in acute cerebral ischemia.